This invention relates, in general, to semiconductor devices and more particularly to a backside metallization scheme for semiconductor devices.
Backside metallization schemes of this type generally must have three main properties. First, the layer disposed directly on a wafer must adhere to that wafer. Second, the outer surface of the metallization scheme must be solderable so that the semiconductor device may be bonded to lead frames and the like. Third, the metallization scheme itself must not come apart. Existing metallization schemes encounter various problems in regards to the above properties.
One existing metallization scheme includes a chromium layer disposed directly on the backside of a wafer, an intermediate layer of silver and chromium disposed on the chromium layer and a silver layer disposed on the intermediate layer. The silver layer exhibits excellent solderability and the scheme, in general, does not come apart. However, while the chromium layer adheres well to the backside of silicon wafers having a relatively rough surface (approximately 300 grit equivalency), the adhesion of the chromium layer to the silicon wafer backside decreases as the surface becomes finer. When a chromium layer is disposed directly on the backside of a silicon wafer having a 2000 grit equivalency, the adhesion of the chromium to the silicon is extremely poor.
Another metallization scheme of this type includes forming a vanadium layer directly on a wafer, a nickel layer on the vanadium layer and then, an optional gold-germanium layer may be formed on the nickel layer. Vanadium has excellent adhesion properties in conjunction with silicon. However, the nickel layer does not have good solderability characteristics in this context. Further, the optional gold-germanium layer does not have good solderability characteristics in this context and is very expensive to employ. Therefore, it would be highly desirable to have a metallization scheme that has good adherence to smooth wafers, excellent solderability characteristics and that will not come apart during the lifetime of the semiconductor device.